1. Field of the Invention
The present invention relates generally to communication systems, and more particularly to such systems, for simultaneously exchanging signals.
2. Description of the Prior Art
Prior art duplex communication systems which provide for simultaneously transmitting and receiving signals have certain disadvantages. The most common of the prior art systems utilize radio frequencies and comprises two radio stations, each of which transmits signals on a different radio frequency channel. Thus the transmitter and receiver of each station operate on different frequencies. Utilizing two radio frequency channels is undesirable in that it requires an additional channel in a crowded radio frequency spectrum. In addition, the use of two radio frequency channels at each station predisposes the station to overload of its receiver by its own transmitter of the same station.
Another prior art duplex communication system employs compression and expansion of signals in the time domain along with transmission and reception at a rapid rate in order to provide apparently simultaneous signal transmission and reception. This system requires highly specialized magnetic recording and reproduction devices to effect the compression and expansion. An example of this system is shown in U.S. Pat. No. 3,827,052, which issued on July 30, 1974 to Tanaka for "Simultaneous Radio Communication System Between Two Stations".
Still another such prior art duplex communication system which is similar to the system of U.S. Pat. No. 3,827,052 is shown in U.S. Pat. No. 4,086,429, which issued on Apr. 25, 1978 to Kotezawa et al. for "Synchronizing System for Use in Telecommunication". This system utilizes pairs of bucket brigade devices (BBD's) to compress and expand the signals. These devices function as analog shift registers. A signal is loaded first into one BBD, then into a second BBD. As the second BBD is loaded, the first BBD is unloaded at either twice the loading speed to effect compression or is unloaded at half the loading speed to effect expansion. A BBD is a clocked mechanism, as with a digital shift register. The clock frequency controls the loading and unloading speeds. Preferably four BBD's are utilized at each of two radio stations. Since BBD's are quite expensive, this system may be economically untenable. Further, synchronization of the two stations can be difficult to achieve. One system for synchronization is shown in U.S. Pat. No. 4,086,429 referred to above.
Thus, there is a need for a duplex communication system which provides simultaneous exchange of signals without the time compression and expansion and the synchronization difficulties inherent thereto.